• RbI is the most effective additive to mitigate PbI2 precipitation caused by Pb(SCN)2 while maintaining large grains.
• Rb ions segregate at grain boundaries during crystallization and Ostwald ripening, promoting slow grain growth and reduced non-radiative recombination.
• Blade-coated wide-bandgap perovskite solar cells in ambient air achieved a certified power conversion efficiency of 23%, among the highest reported.
• The scalable fabrication strategy enables stable MA-free WBG PSCs under ambient conditions, advancing commercialization of tandem solar cells.